The present invention is useful in optical systems, and in particular, in optical systems that create a perceivable image of an object by the transmission of radiation through a lens. While not so limited, the present invention has particular application to light microscopes that use an iris diaphragm to enhance certain characteristics of the image created in the microscope.
A standard element of almost all modem light microscopes is a variable diameter (diaphragm) iris disposed at the aperture of the objective lens or at a plane conjugate to the objective aperture to control the area of the lens that passes light. As used herein, the term xe2x80x9cobjective aperturexe2x80x9d shall mean, and will be understood by those skilled in the art to be, a lens aperture (or back focal plane) or any aperture conjugate to a lens aperture.
By effectively decreasing the area (diameter) of the lens that passes light by closing down the iris diaphragm, the contrast and depth of field of the perceived image is increased, while light intensity and resolution are decreased. Iris diaphragms used for the foregoing purpose have not changed materially for many decades. Investigators using a microscope with such an iris are able to adjust the iris to obtain the best combination of contrast, depth of field and resolution for the particular investigation being conducted.
The degradation of resolution by increasing contrast and depth of field with an iris diaphragm is a result of the iris blocking light from the outer portions of the lens, thereby reducing the working numerical aperture (NA) of the lens and hence reducing the higher order image-forming wavelets that pass through the lens. This phenomenon is explained in detail in Dr. Greenberg""s U.S. Pat. No. 5,345,333.
The present invention provides an aperture mask for an image-forming lens that not only increases image contrast and depth of field, but does so without compromising image resolution. In some directions of observation, the image resolution is even increased, along with increased contrast and depth of field. In addition, the aperture mask produces highlights and shadows of structures within the object being observed, producing an image with enhanced definition.
While the invention is described, for the most part, with reference to light microscopes, it will be immediately recognized by those skilled in the art that the invention is useful in any optical system in which radiation is transmitted through a lens to form an image of an object to be perceived by the human eye or some other detection system.
Rather than using a mask having a circular, co-axial, variable-diameter aperture, as taught by the prior art, the present invention teaches the use of a mask aperture in the shape of a variable-size sector of a circle (wedge-shaped), with the vertex of the sector at the lens optical axis and its two defining sides which diverge from the vertex extending to or beyond the periphery of the lens (and thus, the lens aperture). The sector-shaped mask aperture with its vertex at the lens optical axis permits transmission of light through a portion of the objective lens that includes the center of the lens and its periphery so that the higher order image-forming wavelets that would be transmitted by the unmasked lens are included in the perceived image formed with the masked lens, thereby eliminating the cause of resolution degradation. By varying the angle between the defining sides of the sector-shaped aperture, the size of the aperture and the shape of the beam of light that is transmitted through the lens aperture is varied, thereby varying contrast and depth of field. The variability of the aperture permits the operator to finely tune the system to create the best match between the object under investigation and the optical system imaging that object.
The terms xe2x80x9csectorxe2x80x9d or xe2x80x9csector of a circlexe2x80x9d are used herein to mean that portion of a circle which includes an arc of the circle and the center of the circle. The terms xe2x80x9caperturexe2x80x9d or xe2x80x9copening,xe2x80x9d as used herein, mean light transmissive, as opposed to opaque, and do not necessarily mean without physical structure.
Because of the particular geometry of a sector of a circle, a mask having a variable-size, sector-shaped aperture can be used with lenses of different numerical apertures and magnifications.
Another advantage of the present invention is that the light beam which passes through the lens through the sector-shaped aperture of the mask is oblique with reference to the optical axis of the lens, thus having functional capabilities well beyond those of a co-axial beam emanating from a standard prior art circular, co-axial, variable-diaphragm iris.
By positioning the sector-shaped aperture of the mask of the present invention at different locations about the optical axis of the objective lens, the object is viewed from different directions or oblique angles relative to the optical axis. The degree of obliquity is further controlled by the angular opening of the sector-shaped aperture. For example, a 90-degree sector will produce a greater angle of obliquity than a 180-degree sector. By so observing an object from different directions, it is possible to differentiate between those elements of the image of the object that are at the focal plane of the lens and those that are above and below the focal plane. As the image of the object is viewed from different directions by changing the angular orientation of the sector-shaped aperture, those elements of the image that are above and below the focal plane appear to change their location relative to the frame of the image, while those elements of the image of the object which are at the focal plane remain stationary. It being well within the art to select out those elements of the image that change their location from those that do not, it is possible, using the present invention, to obtain a focal plane-specific image. By changing the location of the focal plane within the object. It is possible to achieve a number of focal plane-specific images of the object, and from those images, construct a 3-D model of the object in a manner well known to those skilled in the art of confocal microscopes.
Accordingly, it is an object of the present invention to provide a new and improved aperture mask for a lens to increase the contrast, definition and depth of field of an image of an object without decreasing resolution.
It is a further object of the present invention to provide an improved aperture mask for a lens which permits the object to be viewed from different angles of view.
It is another object of the present invention to utilize a sector-shaped mask aperture in a microscope having an objective lens to obtain a plurality of focal plane-specific images of an object from which can be constructed a 3-D model of the object.
Another object of the invention is to provide an improved and universal mask that functions equally with lenses of different numerical apertures and magnifications.
Other advantages and objects of the invention will be apparent to those skilled in the art from the description of the invention which follows with reference to the following drawings.